The present invention relates to terminations of high voltage cables and, particularly, to the voltage stress control cones used for modifying the voltage gradients at the terminations.
The voltage stress control cone modifies the voltage stress distribution at the end of a cable to protect the dielectric insulation from degradation and eventual failure due to the high voltage stress between the electrically conducting core of the cable and the adjacent elements at ground potential forming a part of the end termination. The cone is normally located a given distance from the end of the cable. The distance is determined as a function of the flashover characteristics of the environment in which the cone is placed.
For cables currently available for use below 35 kv, stress control cones can usually be premolded in a factory and slipped over the cable in the field. Such premolded voltage stress control cones are acceptable also for cables rated as high as 35 kV when such cables have relatively thick insulation walls with corresponding low voltage stresses at the outer surface. However, slip-on premolded voltage stress control cones cannot be used with those extruded cables that, for the same voltage rating, have reduced insulation wall thicknesses and, therefore, significantly higher voltage stresses at their outer surfaces, nor can they be used for significantly higher voltage cables.
In Bahder et al. U.S. Pat. No. 4,365,947 issued Dec. 28, 1982, there is described a procedure and apparatus for molding voltage stress control cones in situ on the terminations of insulated high voltage power cables. The object of the invention in said Bahder et al. patent is to assure intimate contact between the cable insulation and the voltage stress control cone. After baring the insulation and insulation shield, the cable is wrapped with an insulating tape to form a preliminary blank roughly conforming to the final geometry desired for the stress control cone. This blank is then surrounded by a mold structure for applying heat and pressure to the blank in order to compress the material to the desired geometry and bind it to the underlying cable structure. A critical part of the process is to assure good contact between the insulation shield and insulation of the cable, on the one hand, and the corresponding element constituting the stress control cone, on the other hand. No separation or voids can be tolerated at the interfaces, for such cavities would give rise to partial discharge of high voltage stresses which discharge would lead eventually to breakdown of the system. The problem becomes severe when the average operating voltage stress of the cable insulation reaches or goes above about 150 volts per mil.
It should be readily apparent that hand wrapping of tapes requires considerable care and is tedious, particularly in cold climates. Attempts have been made to use slip-on premolded voltage stress control cones on high voltage cables, i.e., above 35 kV, but the cone must be installed at a substantial distance from the end, the distance being a function of the voltage rating and environment surrounding the voltage stress control cone. For example, in the case of a 138 kV XLPE (cross-linked polyethylene) cable, where the space between the stress control cone and the termination housing is filled with silicone oil, the stress control cone must be located approximately 50 inches from the end of the cable. To install a premolded cone it is necessary to move it over the cable for this substantial distance. Moving a relatively long element with a very tight fit over this distance and over a comparatively high friction surface becomes very difficult if not impossible. Consequently, the stress control cones have been made of resilient materials having inside diameters slightly larger than the outside diameter over which they have to slide. This then necessitates the use of mechanical compression devices to improve the contact between the adjacent surfaces. Satisfactory contact is not readily obtained. Moreover, in order to obtain the necessary resiliency, the premolded stress control cone has been made of a material having different characteristics than that of the cable insulation about which it is to be installed. This usually results in the specific inductive capacitance being different from the cable insulation and, therefore, gives rise to an electrical field distortion resulting in an irregular voltage stress distribution which may result in concentrating higher voltage stresses on the weakest member.
While the method and apparatus described in said Bahder et al. patent is able to provide a reliable voltage stress control cone, such apparatus is bulky and requires considerable time to prepare the cable termination. A significant disadvantage to such system is that if for any reason something goes wrong in the molding operation, e.g., loss of power or unsatisfactory fusing of the cross-linkable tapes, the entire cable on which the molding has been executed must be scrapped. Since such cable is usually cut to its exact length, it becomes necessary to install a joint and replace a section of the cable in order to apply a new stress control cone. This is time consuming and costly.
It is, therefore, an object of the present invention to provide an improved voltage stress control cone that is convenient to apply to the termination of a high voltage cable near the end thereof and which consistently establishes the required intimate union with the underlying cable insulation.